1. Field of the Invention
The invention is based on a piston pump for delivering a fluid, which is used in particular in vehicle brake systems.
2. Description of the Prior Art
In vehicles with hydraulic or electrohydraulic vehicle brake systems, such piston pumps are preferably used as return feed pumps, for selectively lowering or raising a brake pressure in the wheel brake cylinders, and as a result the brake pressure in the wheel brake cylinders can be regulated. Such regulation can be performed for instance in an anti-lock brake system (ABS), a traction control system (TC system), and an electronic stability program, and so forth. FIGS. 1 through 3 show a conventional piston pump that is used in a vehicle brake system. As can be seen from FIGS. 1 through 3, a conventional piston pump has a piston assembly 2, which has a first piston element 2.1, with a sealing element 13, and a second piston element 2.2, an inlet valve 5, an outlet valve 6, and a cylinder 8. The inlet valve 5 is embodied as a check valve and includes a cage element 11, in which an inlet valve spring 5.2 and an inlet valve sealing element 5.3 are disposed, and the inlet valve sealing element 5.3 is embodied for instance as a sealing disk, which can cooperate sealingly with a corresponding inlet valve seat 5.1 that is disposed on the second piston element 2.2, and the second piston element 2.2 is connected by non-positive engagement to the cage element 11. The outlet valve 6 is likewise embodied as a spring-loaded check valve and is disposed in a cap element 12. The outlet valve 6 is opened when a pressure in a compression chamber 8 is greater than a spring force, acting on an outlet valve sealing element 6.2 of the outlet valve 6 of an outlet valve spring 6.3, as a result of which the outlet valve sealing element 6.2 is pressed out of an outlet valve seat 6.1 disposed at an outlet opening 8.3 of the cylinder 8.
During an intake stroke of the piston assembly 2, fluid is aspirated radially via a filter sleeve 9 and transverse bores 3 disposed in the first piston element 2.1 and is carried, via the longitudinal bore 4, corresponding to the transverse bores 3, in the second piston element 2.2 and via the opened inlet valve 5 into the compression chamber 8.1, which is disposed in the cylinder 8 between the inlet valve 5 and the outlet valve 6. Once top dead center is reached, the direction of motion of the piston group 2 is reversed, so that the second piston element 2.2 having the inlet valve seat 5.1 is pressed sealingly against the inlet valve sealing element 5.3, via the first piston element 2.1 driven by an eccentric element 14 disposed in an eccentric chamber 15, and the inlet valve 5 is closed. Now, a pressure buildup takes place in the compression chamber 8.1 until such time as the pressure in the compression chamber 8.1 is greater than the spring force of the outlet valve 6, as a result of which the fluid that is under pressure is conducted via an outlet opening 8.3 and the opened outlet valve 6 from the compression chamber 8.1 into an outlet line, not shown.
After bottom dead center is reached, the direction of motion of the piston assembly 2 reverses again, so that the outlet valve 6 closes again, and the intake stroke begins again; a restoring force F2 of a restoring spring 10, disposed in the compression chamber 8.1 and guided by a cylinder wall 8.4, which spring is embodied for instance as a spiral spring with polished end windings and is braced on a cylinder bottom 8.2 and on the cage element 11, acts against the cage element 11 of the inlet valve 5 and thus against the second piston element 2.2, in order to move the piston assembly 2 back in the direction of top dead center. During operation, axial forces Fl and F2, which on the one hand are effected by the eccentric element 14 and introduced via the first piston element 2.1, and on the other are effected by the restoring spring 10 and introduced via the cage element 11, and radial forces F3, which are generated by the prevailing system pressure, therefore act on the second piston element 2.2, which has the inlet valve seat 5.1. The second piston element 2.2 embodied as a valve seat is therefore produced as a stable component, preferably as a metal component, and can be weakened only conditionally by intake bores in the form of transverse bores 3. The transverse bores are therefore made in the first piston element 2.1 resulting in a relatively long in let region embodied as the longitudinal bore 4.